The invention relates to the fields of thin film arrays, semiconductor processing and microthruster ignition. More particularly, the present invention relates to semiconductor processes and structures for addressing and reading thin film cell arrays well suited for igniting and interrogating semiconductor microthruster cell arrays.
Existing cell elements, such as individual heating, pyrotechnic, thermionic, or field emitter elements disposed in an array need to be selectively addressed and activated. Electrically addressable arrays of elements using a suitable addressing scheme have been used in solid state memories. These individually addressable cells require extensive addressing connections that necessitate complicated routing during semiconductor processing. One method of addressing individual cells is to connect a pair of wires to each cell. This addressing scheme requires 2n2 addressing wires for an array of nxn cells. A common ground wire may be used to reduce the total to n2+1 leads. Diodes have been used with addressing lines for isolated addressing. While diodes built from polycrystalline silicon films have been known for years, polysilicon diodes are not widely used because of poor reverse leakage characteristics. Crystalline silicon has been used to build electrically nonlinear elements, such as isolation diodes and complex transistors fabricated in a single crystal semiconductor substrate. These nonlinear elements have been used for selective addressing of array elements and cells.
In one application, a large array of microthruster cells, each containing heat-sensitive combustible propellant, needs to be individually addressed and ignited without igniting or otherwise damaging neighboring cells. This isolated cell combustion disadvantageously requires extensive addressing lines that can be damaged and open circuited when, for example, a pyrotechnic cell is ignited resulting in a loss of addressability to a damaged neighboring unignited cell. A further disadvantage is an inability to interrogate a pyrotechnic cell to determine if the cell was properly ignited after an ignition command due to destructive combustion. The selecting and applying power to a single element of a large array of microthruster cells each containing heat-sensitive combustible propellant may not be effectively controlled without powering or disturbing neighboring cells. After the cells are ignited and destroyed by the combustion process, there is no addressing method for interrogating the ignited cells to determine whether the cells have been fired or not. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide addressing lines for selectively addressing cells within an array of cells using diode isolation and heating resistors.
Another object of the invention is to provide addressing lines for selectively addressing cells within an array of cells using diode isolation and heating resistors using a single layer of polysilicon and a single layer of metal.
Yet another object of the invention is to provide addressing lines for selectively addressing cells within an array of cells using diode isolation and heating resistors using a single layer of polysilicon and a single layer of metal for connecting a plurality of cell elements within each cell of the array of cells.
Still another object of the invention is to provide a large array of microthruster cells containing heat sensitive combustible propellant that are individually addressed and ignited without igniting or otherwise damaging neighboring cells.
A further object of the invention is to provide a method for interrogating cells, such as combustible cells, to determine whether the cells have been previously fired.
Still a further object of the present invention is to reduce the number of addressing lines required for selective firing and interrogation of any individual cell in an array of pyrotechnic cells well suited for controlled and monitored microthrusting.
The present invention is directed to an array of uniquely addressable cells having one or more cell elements. Addressing the cells relies upon individual diode isolation for selective addressing, firing and interrogating any one of the cells using a single thin film of polycrystalline silicon. The use of an isolating diode in an addressing structure enables individual addressing of micron sized pyrotechnic elements, cells or other microelectromechanical (MEMS) devices. The addressing method is used for addressing an array of nxn cells with only 2n leads so as to simplify the manufacturing processes with efficient use of silicon area and weight well suited for addressing microthruster arrays having many cells. In the preferred form, power is applied to a selected cell in an array of cells using address lines in x-and y-directions. A thin film polysilicon diode array allows a single cell to be isolated and powered. The diode array can be manufactured using conventional photolithographic or screen printing technology and can be placed on any insulating surface. The manufacturing process is compatible with conventional MEMS systems and semiconductor processing, and the diode array can therefore be built on the same substrate material as conventional MEMS or integrated circuits.
The addressing method enables selective interrogation of individual fired pyrotechnic cells to determine whether the individual cells have been previously addressed and ignited. When ignited, the individual cell is destroyed during the combustion process that open circuits the addressed cell connections. Addressing lines are preserved by locating them between fuel cells outside the combustion zone. Current sensing, using for example a current sensing resistor in the addressing lines, can be used to determine whether a cell has been previously fired. Polysilicon diode leakage performance is sufficient to selectively address, fire and interrogate the individual cells. Polysilicon thin films can be used for addressing and firing individual cells using simple polysilicon diodes compatible with MEMS technology for process compatible integration with MEMS devices typically manufactured in polysilicon. The use of polysilicon diodes is particularly advantageous to MEMS manufacturing because polysilicon is compatible with standard MEMS surface micromachining processes as well as integrated circuit elements. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.